A boiling water reactor (BWR) conventionally includes in serial flow communication a lower plenum, a reactor core, an upper plenum, a steam separator, and a steam dryer disposed within a reactor pressure vessel. Typically surrounding the core is an annular shroud spaced radially inwardly from the pressure vessel to define an annular downcomer. The pressure vessel is partially filled with a reactor coolant, such as water, to a level above the reactor core. The reactor coolant enters the core from the lower plenum and is heated thereby which decreases its density and therefore causes it to rise upwardly. The water is heated by the core for generating steam which is separated from moisture in the steam separator and the steam dryer, and is discharged from the pressure vessel through an outlet nozzle for flow to a conventional steam turbine for powering an electrical generator for example.
Condensed steam from the turbine is pressurized by a conventional feedwater pump and returned to the pressure vessel through a conventional feedwater sparger disposed therein for mixing in the downcomer the relatively cool feedwater with the hot reactor coolant. The reactor coolant is thereby cooled which increases its density, and it thereby falls by gravity in the downcomer to the lower plenum for completing the recirculation loop. Since this natural, gravity-driven recirculation of the reactor coolant, based on the difference in temperature of the reactor coolant in the downcomer and in the core, has a relatively low flowrate, reactors typically include systems for forcing recirculation of the coolant flow within the vessel.
For example, conventional jet pumps include a nozzle for ejecting a driving fluid as a jet into a mixer and in turn into a diffuser which draws by suction into the mixer a portion of the reactor coolant in the downcomer for providing forced recirculation. The nozzle is provided with the driving fluid from a driving pump located externally of the pressure vessel, and through corresponding supply pipes extending through the pressure vessel in flow communication with respective jet pumps. An outlet from the pressure vessel is joined to the driving pump by a suitable supply pipe for providing thereto a portion of the reactor coolant for being pressurized.
Such jet pumps are generally undesirable since they require the external driving pump and piping loops which add to the complexity of the system, increase the number of welds required in manufacturing the system, and must be suitably configured for containing radiation due to the recirculating coolant therein. Furthermore, the inlets and outlet required in the vessel for channeling the flow to and from the jet pumps also provide additional sources for potential leakage of the reactor coolant, and are typically located below the top of the reactor core which could uncover the reactor core in a leakage incident.
Whereas the jet pumps are fluid-driven and require the driving pump and external piping, conventional reactor internal pumps (RIPs) are impeller-driven and may be contained within the reactor pressure vessel thusly eliminating the external loops and the driving pump used for jet pumps, and the attendant problems associated therewith. The RIPs also provide improved control of the flow of reactor coolant through the core by using conventional adjustable speed drives.
However, tradeoffs are required in a RIP recirculation system. For example, in a pump trip situation where all of the RIPs are rendered inoperable, the RIPs provide a large forward resistance to the natural recirculation of the reactor coolant down the downcomer which reduces the core inlet flow substantially and may result in unstable, or oscillatory, flow in the reactor core unless the core power level is reduced, for example, by inserting control rods. Accordingly, contingency measures are typically included in the design to exclude unstable operation, and the complexity of the overall system is thereby increased.
Furthermore, upon pump trip, the RIPs coast down very rapidly due to their low rotary inertia and therefore a similarly rapid reduction in core flow occurs. Upon occurrence of the pump trip, the reactor is also scrammed, but, however, the heating ability of the nuclear fuel cannot be instantaneously stopped and therefore the reactor coolant continues to be heated thereby. With the reduced core flow, transition boiling may occur where the reactor coolant is heated for forming a steam film around the fuel assemblies instead of nucleate bubbles which can overheat the cladding surrounding the fuel rods in the fuel assemblies potentially leading to damage thereof or shortened lifetimes.
In order to prevent the occurrence of transition boiling in a pump trip situation, redundant and independent power supplies may be employed to reduce the probability of an all pump trip, or inertia may be built into the system by adding mass to the RIPs. Or, in another embodiment, conventional motor-generator sets may be used as a power supply source for some of the RIPs. The electrically driven motor-generator set in turn provides electricity to the RIPs, and, in the event of a pump trip, the motor-generator sets have sufficient additional inertia for slowing the coast down of the RIPs connected thereto for reducing, or eliminating, transition boiling.